1. Field of the Invention
The present invention relates to drilling subsea. In particular, the present invention relates to a system and method for sealingly positioning a rotating control head in a subsea housing.
2. Description of the Related Art
Marine risers extending from a wellhead fixed on the floor of an ocean have been used to circulate drilling fluid back to a structure or rig. The riser must be large enough in internal diameter to accommodate the largest bit and pipe that will be used in drilling a borehole into the floor of the ocean. Conventional risers now have internal diameters of 19½ inches, though other diameters can be used.
An example of a marine riser and some of the associated drilling components, such as shown in FIG. 1, is proposed in U.S. Pat. No. 4,626,135, assigned on its face to the Hydril Company, which is incorporated herein by reference for all purposes. Since the riser R is fixedly connected between a floating structure or rig S and the wellhead W, as proposed in the '135 Hydril patent, a conventional slip or telescopic joint SJ, comprising an outer barrel OB and an inner barrel IB with a pressure seal therebetween, is used to compensate for the relative vertical movement or heave between the floating rig and the fixed riser. A diverter D has been connected between the top inner barrel IB of the slip joint SJ and the floating structure or rig S to control gas accumulations in the marine riser R or low pressure formation gas from venting to the rig floor F. A ball joint BJ above the diverter D compensates for other relative movement (horizontal and rotational) or pitch and roll of the floating structure S and the fixed riser R.
The diverter D can use a rigid diverter line DL extending radially outwardly from the side of the diverter housing to communicate drilling fluid or mud from the riser R to a choke manifold CM, shale shaker SS or other drilling fluid receiving device. Above the diverter D is the rigid flowline RF, shown in FIG. 1, configured to communicate with the mud pit MP. If the drilling fluid is open to atmospheric pressure at the bell-nipple in the rig floor F, the desired drilling fluid receiving device must be limited by an equal height or level on the structure S or, if desired, pumped by a pump to a higher level. While the shale shaker SS and mud pits MP are shown schematically in FIG. 1, if a bell-nipple were at the rig floor F level and the mud return system was under minimal operating pressure, these fluid receiving devices may have to be located at a level below the rig floor F for proper operation. Since the choke manifold CM and separator MB are used when the well is circulated under pressure, they do not need to be below the bell nipple.
As also shown in FIG. 1, a conventional flexible choke line CL has been configured to communicate with choke manifold CM. The drilling fluid then can flow from the choke manifold CM to a mud-gas buster or separator MB and a flare line (not shown). The drilling fluid can then be discharged to a shale shaker SS, and mud pits MP. In addition to a choke line CL and kill line KL, a booster line BL can be used.
In the past, when drilling in deepwater with a marine riser, the riser has not been pressurized by mechanical devices during normal operations. The only pressure induced by the rig operator and contained by the riser is that generated by the density of the drilling mud held in the riser (hydrostatic pressure). During some operations, gas can unintentionally enter the riser from the wellbore. If this happens, the gas will move up the riser and expand. As the gas expands, it will displace mud, and the riser will “unload.” This unloading process can be quite violent and can pose a significant fire risk when gas reaches the surface of the floating structure via the bell-nipple at the rig floor F. As discussed above, the riser diverter D, as shown in FIG. 1, is intended to convey this mud and gas away from the rig floor F when activated. However, diverters are not used during normal drilling operations and are generally only activated when indications of gas in the riser are observed. The '135 Hydril patent has proposed a gas handler annular blowout preventer GH, such as shown in FIG. 1, to be installed in the riser R below the riser slip joint SJ. Like the conventional diverter D, the gas handler annular blowout preventer GH is activated only when needed, but instead of simply providing a safe flow path for mud and gas away from the rig floor F, the gas handler annular blowout provider GH can be used to hold limited pressure on the riser R and control the riser unloading process. An auxiliary choke line ACL is used to circulate mud from the riser R via the gas handler annular blowout preventer GH to a choke manifold CM on the rig.
Recently, the advantages of using underbalanced drilling, particularly in mature geological deepwater environments, have become known. Deepwater is considered to be between 3,000 to 7,500 feet deep and ultra deepwater is considered to be 7,500 to 10,000 feet deep. Rotating control heads, such as disclosed in U.S. Pat. No. 5,662,181, have provided a dependable seal between a rotating pipe and the riser while drilling operations are being conducted. U.S. Pat. No. 6,138,774, entitled “Method and Apparatus for Drilling a Borehole into a Subsea Abnormal Pore Pressure Environment,” proposes the use of a rotating control head for overbalanced drilling of a borehole through subsea geological formations. That is, the fluid pressure inside of the borehole is maintained equal to or greater than the pore pressure in the surrounding geological formations using a fluid that is of insufficient density to generate a borehole pressure greater than the surrounding geological formation's pore pressures without pressurization of the borehole fluid. U.S. Pat. No. 6,263,982 proposes an underbalanced drilling concept of using a rotating control head to seal a marine riser while drilling in the floor of an ocean using a rotatable pipe from a floating structure. U.S. Pat. Nos. 5,662,181; 6,138,774; and 6,263,982, which are assigned to the assignee of the present invention, are incorporated herein by reference for all purposes. Additionally, provisional application Ser. No. 60/122,350, filed Mar. 2, 1999, entitled “Concepts for the Application of Rotating Control Head Technology to Deepwater Drilling Operations” is incorporated herein by reference for all purposes.
It has also been known in the past to use a dual density mud system to control formations exposed in the open borehole. See Feasibility Study of a Dual Density Mud System for Deepwater Drilling Operations by Clovis A. Lopes and Adam T. Bourgoyne, Jr., © 1997 Offshore Technology Conference. As a high density mud is circulated from the ocean floor back to the rig, gas is proposed in this May of 1997 paper to be injected into the mud column at or near the ocean floor to lower the mud density. However, hydrostatic control of abnormal formation pressure is proposed to be maintained by a weighted mud system that is not gas-cut below the seafloor. Such a dual density mud system is proposed to reduce drilling costs by reducing the number of casing strings required to drill the well and by reducing the diameter requirements of the marine riser and subsea blowout preventers. This dual density mud system is similar to a mud nitrification system, where nitrogen is used to lower mud density, in that formation fluid is not necessarily produced during the drilling process.
U.S. Pat. No. 4,813,495 proposes an alternative to the conventional drilling method and apparatus of FIG. 1 by using a subsea rotating control head in conjunction with a subsea pump that returns the drilling fluid to a drilling vessel. Since the drilling fluid is returned to the drilling vessel, a fluid with additives may economically be used for continuous drilling operations. ('495 patent, col. 6, ln. 15 to col. 7, ln. 24) Therefore, the '495 patent moves the base line for measuring pressure gradient from the sea surface to the mudline of the sea floor ('495 patent, col. 1, lns. 31-34). This change in positioning of the base line removes the weight of the drilling fluid or hydrostatic pressure contained in a conventional riser from the formation. This objective is achieved by taking the fluid or mud returns at the mudline and pumping them to the surface rather than requiring the mud returns to be forced upward through the riser by the downward pressure of the mud column ('495 patent, col. 1, lns. 35-40).
U.S. Pat. No. 4,836,289 proposes a method and apparatus for performing wire line operations in a well comprising a wire line lubricator assembly, which includes a centrally-bored tubular mandrel. A lower tubular extension is attached to the mandrel for extension into an annular blowout preventer. The annular blowout preventer is stated to remain open at all times during wire line operations, except for the testing of the lubricator assembly or upon encountering excessive well pressures. ('289 patent, col. 7, lns. 53-62) The lower end of the lower tubular extension is provided with an enlarged centralizing portion, the external diameter of which is greater than the external diameter of the lower tubular extension, but less than the internal diameter of the bore of the bell nipple flange member. The wireline operation system of the '289 patent does not teach, suggest or provide any motivation for use a rotating control head, much less teach, suggest, or provide any motivation for sealing an annular blowout preventer with the lower tubular extension while drilling.
In cases where reasonable amounts of gas and small amounts of oil and water are produced while drilling underbalanced for a small portion of the well, it would be desirable to use conventional rig equipment, as shown in FIG. 1, in combination with a rotating control head, to control the pressure applied to the well while drilling. Therefore, a system and method for sealing with a subsea housing including, but not limited to, a blowout preventer while drilling in deepwater or ultra deepwater that would allow a quick rig-up and release using conventional pressure containment equipment would be desirable. In particular, a system that provides sealing of the riser at any predetermined location, or, alternatively, is capable of sealing the blowout preventer while rotating the pipe, where the seal could be relatively quickly installed, and quickly removed, would be desirable.
Conventional rotating control head assemblies have been sealed with a subsea housing using active sealing mechanisms in the subsea housing. Additionally, conventional rotating control head assemblies, such as proposed by U.S. Pat. No. 6,230,824, assigned on its face to the Hydril Company, have used powered latching mechanisms in the subsea housing to position the rotating control head. A system and method that would eliminate the need for powered mechanisms in the subsea housing would be desirable because the subsea housing can remain bolted in place in the marine riser for many months, allowing moving parts in the subsea housing to corrode or be damaged.
Additionally, the use of a rotating control head assembly in a dual-density drilling operation can incur problems caused by excess pressure in either one of the two fluids. The ability to relieve excess pressure in either fluid would provide safety and environmental improvements. For example, if a return line to a subsea mud pump plugs while mud is being pumped into the borehole, an overpressure situation could cause a blowout of the borehole. Because dual-density drilling can involve varying pressure differentials, an adjustable overpressure relief technique has been desired.
Another problem with conventional drilling techniques is that moving of a rotating control head within the marine riser by tripping in hole (TIH) or pulling out of hole (POOH) can cause undesirable surging or swabbing effects, respectively, within the well. Further, in the case of problems within the well, a desirable mechanism should provide a “fail safe” feature to allow removal the rotating control head upon application of a predetermined force.